The present invention relates to an automatic transmission, in which first and second transmission assemblies capable of controlling shifts individually are connected in tandem, and, more particularly, to a shift control system for use with the automatic transmission, in which gear stage trains to be adopted are changed in dependence upon the running conditions.
There is known an automatic transmission in which a first transmission assembly capable of shifting first to third speeds and a second transmission assembly capable of shifting two higher and lower speeds are connected in tandem. This automatic transmission is exemplified by Japanese Patent Laid-Open No. 61-244957 where six forward speeds can be shifted.
Here, the number of gear stages required for the actual running is mostly satisfied by four or five. As a result, the automatic transmission capable of setting totally six gear stages, as disclosed in the above-specified Laid-Open, is disadvantageous in that the shifts are frequently caused with a slight change in the vehicle speed. Since, in the disclosed automatic transmission, the first transmission assembly and the second transmission assembly have their shifts separately controlled, both of them will may happen to be simultaneously shifted such that one is brought into an upshift and the other into a downshift, in this case, another disadvantage arises in that the controls must be made complex for reducing the shifting shocks or in that the shocks are increased because the shift timing is not adequately controlled.
A system for eliminating those disadvantages is disclosed in Japanese Paten: Laid-Open Mo. 60-116953. In the automatic transmission capable of selecting a power pattern and an economy pattern by means of a pattern select switch, according to the disclosure, the system is constructed to have different gear stage trains in dependence upon the power and economy patterns selected, in this system, the automatic transmission is shifted sequentially in the order of 1st, 2nd . . . and so on speeds in accordance with the vehicle speed or the engine load, if the power pattern is selected. From the gear stage trains for the economy pattern, there is eliminated such a specific gear stage as is difficult to control the shifts: from the 2nd to 3rd speeds, for example. As a result, the shifts are accomplished in the order of the 1st, 2nd, 4th, . . . , and so on speeds.
The automatic transmission thus enabled to select the gear stage trains manually is excellent in that the vehicle has its drivability improved because its power characteristics can be selected by the driver himself.
In the automatic transmission capable of changing the gear stage trains on the basis of the running conditions such as the selected running pattern, a shift is caused by switching the running pattern even if the vehicle speed and/or the engine load are unvaried. Let the case be considered, in which a shift is caused by the change in the running pattern in the automatic transmission in which the individual shift-controlled first and second transmission assemblies are connected in tandem, if the shift is one to be achieved by shifting the first and second transmission assemblies, it is difficult to make proper the timing of the shifts at the individual transmission assemblies. Especially if the first and second transmission assemblies are to be shifted oppositely cf each other, the shift is difficult to smoothen, and the shifting shocks may possibly be great.